It has been remarked to produce hydrocarbons from biomass, and has been desired to provide them as liquid fuels, from the viewpoint of being reproducible, and reducing the amount of carbon dioxide discharged by carbon neutralization.
Presently, as fuels using biomass as a raw material, methyl esters of fatty acids are practically used as diesel fuels. However, the methyl esters of fatty acids produce about 10% of glycerol, a raw material of fats and oils as a by-product, and complete removal of this glycerol is difficult, so that the quality of the fuels is lowered. Also, the methyl esters of fatty acids are disadvantageous in that viscosities are large, from the viewpoint of low-temperature flowability. In addition, the methyl esters of fatty acids have an unsaturated bonding group in a carbon chain, so that the oxidation stability is poor. As described above, the methyl esters of fatty acids are yet disadvantageous in quality.
As hydrocarbon liquid fuels of the next generations, a biomass raw material oil is reacted with hydrogen under high temperatures and high pressures in the presence of a catalyst, to form a hydrocarbon from an alkyl chain of the fats and oils has been considered. However, it is limited to the techniques for production of petroleum fuels that are practiced, in other words, the high-temperature, high-pressure hydrocracking techniques (Patent Publication 1 and Non-Patent Publication 1). According to these publications, high pressures of from 2 to 5 MPa are required, and it can be assumed that the reaction hardly progresses at a normal pressure, so that experimental results at a normal pressure are not given. In Patent Publication 1, it is recommended that a preferred pressure is from 1 to 5 MPa. The studies of the fields to be especially referred is Non-Patent Publication 1, in which experimentations are conducted in which a catalyst Ni or Mo is supported to silica or alumina, a raw material fat or oil is Jatropha oil, and a pressurization fixed bed reaction apparatus (1 to 8 Mpa) is used, but any experimental results at a normal pressure are not given.
Besides the above, it is considered that a fat or oil is gasified, and a hydrocarbon is produced from the gas via Fischer-Tropsch synthesis. These can be referred in the same manner as the steps in the case of the petroleum raw material, which is a high-temperature, high-pressure technique.
The method of hydrocracking a fat or oil in the above publications includes saturating an unsaturated binding group of a fat or oil by hydrogenation to remove oxygen, and at the same time cracking a triglyceride of a fat or oil. In the hydrodeoxygenation reaction, a paraffin-based hydrocarbon, water, propane, or the like is produced from triglyceride and hydrogen in the presence of a catalyst by hydrogenation dehydration reaction, decarbonylation reaction, and decarboxylation reaction, under high temperatures and high pressures.
For example, it is disclosed that using a desulfurization and hydrogenation catalyst as a catalyst and a purified palm oil as a fat or oil, palm oil is cracked at a reaction pressure of 6 MPa and a reaction temperature of 260° C. or higher, to produce 85% or so of a gas oil fraction, 10% of water, and 5% of gas (carbon dioxide, methane, propane). The reaction product obtained is constituted by a linear hydrocarbon having 15 carbon atoms to 18 carbon atoms, having physical properties nearing a gas oil (Non-Patent Publication 2). However, since the N-paraffin is the main component, there is a disadvantage in low-temperature flowability, and the product does not sufficiently satisfy other properties as fuels.
On the other hand, the hydrocracking technique to a hydrocarbon in a petroleum oil is nearly in a realm of perfection and well known to be practiced. According to a text book (Non-Patent Publication 3), a catalyst in which a precious metal such as platinum is added to a zeolite-based solid acid catalyst is used as a cracking catalyst, and naphtha, kerosene, gas oil, or the like is produced in a reaction under high-temperature, high-pressure conditions. During the reaction, besides the cracking of the hydrocarbon chain, cyclized dehydrogenation, dehydrogenation, or isomerization takes place, so that physical properties required for fuels such as calorimetric amount, octane number, and cetane number are given. By the addition of platinum to the catalyst, the carbon formation on the catalyst caused by hydrogen deficiency in various reactions mentioned above is suppressed by high-pressure hydrogen, which makes the life of the catalyst practically durable. However, the perfect carbon formation is not suppressed, so that the life of the catalyst is limited. The developments of the catalyst with longer lives are even more desired.